Obtaining filaments from polyamide acid salts



United States Patent 3,529,051 OBTAINING FILAMENTS FROM POLYAMIDE ACID SALTS Alan Buckley and John David Seddon, Runcorn, England, assignors to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed Aug. 18, 1967, Ser. No. 661,537 Claims priority, application Great Britain, Sept. 6, 1966, 39,805/66 Int. Cl. D01f 7/00 US. Cl. 264184 7 Claims ABSTRACT OF THE DISCLOSURE A method for producing filaments of polyamide acids or salts thereof suitable for conversion to polyimide filaments which is characterised by a filament spinning stage comprising spinning an aqueous solution of an ammonia or organic base salt of a polyamic acid into an aqueous or water miscible coagulant bath which chemically interacts with the ammonia or organic base salt to regenerate the polyamide acid or otherwise convert the salt into an insoluble form and withdrawing from the coagulant bath a coherent filament.

The present invention relates to the production of filaments and fibres from the aqueous solutions of ammonia or organic base salts of polyamide acids. These solutions and their production form the subject of the specification of Belgian Pat. No. 672,175 standing in the name of Imperial Chemical Industries Ltd. which discloses polyamide acids from tetracarboxylic acids capable of form ing dianhydrides and diprimary diamines and discloses that these polyamide acids are capable of being shaped by conventional techniques and may then be converted with loss of water to infusible polyimides having a very useful combination of chemical, thermal and electrical properties.

That specification notes that filaments may be formed from these aqueous solutions by spinning into a non solvent for the polymer, e.g. dioxane, and that the filaments so obtained may be converted to polyimide e.g. by heating. According to the present invention, we have found an improved method of obtaining filaments from these aqueous solutions. This method comprises passing a fine stream of the aqueous solution into a coagulating medium which is capable of converting the soluble ammonia or organic base salt of the polyamide acid into an insoluble form, for example, into the corresponding polyamide acid or metal salt thereof, the coagulating medium being aqueous or water miscible. A filament of the resultant coagulated polyamide acid (say) may then be recovered and converted, if desired, into a polyimide filament. British patent specification No. 1,038,738 should be referred to for information on conversion techniques and methods of processing filaments of polyimide precursors. As stressed in that specification it is very important that the precursor filament should be subject to minimal tension during the conversion stage or stages if filaments of more preferred properties are required.

Monofilaments may be obtained by using a single fine stream of the aqueous solution according to our invention and multi-filamentary fibres or yarns may be formed by spinning a number of fine streams in known manner.

Examples of coagulating media are solutions in which the solutes are stronger acids than the polyamide-acid from which the ammonia or organic base salt is derived or are stronger bases than the ammonia or organic base used to form the said polyamide acid salt, or are metal salts which are capable of undergoing a double-decom- 3,529,051 Patented Sept. 15, 1970 position reaction with the ammonia or organic base salt of the polyamide acid.

In the first case, the free polyamide acid is formed which is insoluble, in the second and third cases another salt of the polyamide acid is formed which must, of course, not be soluble.

Particular examples of these are hydrochloric acid, trichloracetic acid, calcium hydroxide, calcium chloride, ferric chloride, silver nitrate and metal chelates.

In the coagulating media used in our invention, the solutes are dissolved in water or in water-miscible liquids which are non-solvents for the polymer product obtained from the reaction between the solute and the ammonia or organic base polyamide and salt involved. It is sometimes advisable to avoid the use of water because the products obtained by the reaction may be water-sensitive and suffer from hydrolytic degradation. This disadvantage may be avoided to some extent by Washing the water from the coagulated filaments formed by the reaction immediately after formation, e.g. by passing the filaments through a methanol bath, but in general it is easier to replace some or all of the water in the first instance. Alternative solvents must be water-miscible, since they must absorb the water from the aqueous solution of the ammonium or organic base salt, and must also be nonsolvents for the polymeric products obtained by the reaction with the chosen solute. Examples are alcohols, e.g. methanol, and the lower molecular weight esters, e.g. ethyl acetate. Ketones and others such as acetone and dioxane are not suitable as they appear to be partial solvents for the products.

We have obtained our best results from the use of an aqueous methanolic solution of hydrochloric acid.

The filaments, yarns, fibres, etc., obtained by our process consist essentially of polyamide acid or the appropriate metal salt thereof. These filaments etc., may be converted to polyimide, if desired, by heating alone or in the presence of a chemical agent encouraging cyclisation such as (where the polyamide acid is involved) acetic anhydride in pyridine. Where coagulation is effected by metal salt solution, pyrolysis of the coagulated fibre will result in metal residues being dispersed in the polyimide matrix. In the case where a silver salt solution is used for coagulation, pyrolysis will ordinarily cause at least some conversion of the silver residues to the metal. The resulting polyimide may then possess interesting electrical properties.

In general, however, the method of coagulation involving the formation of an insoluble metal salt is the least satisfactory method because the imidisation step releases a non-volatile byproduct and because the presence of the metal species may interfere with polyimide conversion or otherwise give rise to polyimide filaments of inferior properties.

Preferred, ammonium or organic base salts of polyamic acids for use in the present invention are such salts of the polyamic acid derivable from pyromellitic dianhydride and 4,4'-diamino diphenyl ether (or thioether). Preferred amongst organic bases are trialkyl amines, especially triethyl amine and trimethyl amine.

The invention is illustrated by the following examples.

EXAMPLE 1 An gm./litre solution of the triethylamine salt of the polyamide acid derived from 4,4-diaminodiphenyl ether and pyromellitic acid dianhydride was prepared using a mixture of equal volumes of water and isopropanol as the solvent. The polyamide acid had an intrinsic viscosity (measured on a solution of the polymer in dimethyl acetamide at 25 C.) of 0.67. This solution was extruded through a 0.012 inch diameter orifice into a bath containing a solution of volumes of bench concentrated hydrochloric acid in 95 volues of methanol. A fibre was obtained which was allowed to travel about 30 inches through the bath before being wound up at a rate of about 12 ft./minute."

A repeat experiment to obtain a coherent fibre failed when methanol alone was used in the bath.

EXAMPLE 2 The process of Example 1 was repeated using a variety of solutions in the bath. The conditions are tabulated below. Continuous fibres were obtained in all cases.

Example 2D was repeated and the fibre obtained was washed immediately in methanol before Winding up. This resulted in a fibre which showed a reduced tendency to discolour.

EXAMPLE 4 The solution used for spinning and the spinning apparatus were as described in Example 1. The coagulant bath was a 3.5% v./v. solution of commercial concentrated hydrochloric acid in water. The temperature of the coagulant bath was C. and the as-spun filament wind-up rate was 28.6 feet per minute. Draw-down in the coagulant bath was controlled between 1.3 and 2.0x The as-spun filament, after washing with aqueous methanol to remove residual coagulant, was dried in air at 100 C. for 1 hour and thereafter wound into a skein and heated at 300 C. in the absence of imposed tension to convert the polyamide acid into the polyimide. The tenacity of the filament before and after conversion to the polyimide was 0.7 and 2.8 gms./denier respectively. The initial modulus 0 before and after conversion was 32 and 31 respectively. The extension-to-break of the filament before and after conversion was 2.7 and 79% respectively. The polyimide filament could be drawn 1.5x over a curved heated plate held at 550 C. and some improvement in properties, as expected, was apparent.

Tenacity, extension-to-break, and initial modulus values were obtained under the usual conditions: viz at a temperature of 69 F. and relative humidity of 65% and at a rate of extension of 100% per minute, for tenacity and extension-to-break, or 10% per minute for initial modulus. As Instron tensile testing machine was used (Instron is a trade name).

4 EXAMPLE 5 Example 4 was repeated using a 7.5% v'./v. solution of concentrated hydrochloric acid in methanol as coagulant bath and a wind-up rate for the as-spun filament of 20 feet/minute. Tenacity, initial modulus and extension values before and after conversion to polyimide were 1.1 and 1.3 (tenacity), 47 and 31 (initial modulus) and 4.1 and 29 (extension). Once again, further drawing to 1.5 X over a curved heated plate held at 550 C. caused some improvement in properties.

What we claim is:

1. A method for producing filaments of polyamide acids or salts thereof suitable for conversion to polyimide filaments comprising spinning an aqueous solution of an organic base salt of a polyamic acid of a diprimary diamine and a tetracarboxylic acid which is capable of forming a dianhydride into an aqueous or water miscible coagulant bath which chemically interacts with the organic base salt to regenerate the polyamide acid or otherwise convert the salt into an insoluble form and withdrawing from the coagulant bath a coherent filament.

2. A method as claimed in claim 1 wherein the spinning solution is a solution of a trialkylamine salt of the polyamide acid derived from pyromellitic dianhydride and a diamine of the general formula wherein X is a bridging group such as O or S and the phenyl groups are further substituted or are unsubstituted.

3. A method as claimed in claim 1 wherein the coagulant bath is an acid solution and the spun salt is converted into the free polyamide acid.

4. A method for producing polyimide filaments which comprises subjecting a filament of a polyamide acid or derivative thereof produced by a method as claimed in claim 1 to thermal imidisation under a tension not exceeding 0.5 gm./ denier.

5. A method as set forth in claim 4 in which the tens'ion is less than 0.2 gms./ denier.

- 6. A method as set forth in claim 4 including the step of further hot drawing the filament after thermal imidisation.

7. A method for producing polyimide filaments which comprises subjecting to thermal imidisation a filament of a polyamide acid or derivative thereof produced by a method as set forth in claim 2, the coagulant bath being an acid bath.

References Cited UNITED STATES PATENTS 3,440,196 4/ 1969 Boldebuck et a1. 3,440,197 4/1969 Boldebuck et a]. 3,441,640 4/1969 Santangelo 264203 X 3,448,068 6/1969 Holub et al. 3,453,236 7/1969 Culbertson 26029.2 X

JULIUS FROME, Primary Examiner J. H. WOO, Assistant Examiner US. Cl. X.R. 

